Flexible membrane circuits are in widespread usage in a variety of applications. Among these, by way of example, are input devices actuated by finger pressure, for data entry, control setting, etc. Typically, such devices involve a flexible membrane, provided with externally visible graphics, such as number pads, on/off switches and the like, where an operator applies finger pressure to the outside of the flexible membrane, over a selected graphic element, to perform a desired operation. Such flexible membrane circuits are normally constructed using printed circuit principles, and can be relatively complex in nature.
Typical procedures for the manufacture of flexible membrane circuits involve the preparation of polyester films, which are printed with successive layers of electrically functional inks (conductive and dielectric). Typical conductive inks are pigmented with silver or graphite, although other metals such as gold, nickel and copper have also been employed. The conductive ink is printed directly onto the surface of the polyester film in the form of electrical paths that will make up a circuit or switch. A very simple circuit may comprise a single layer of conductive ink, applied in a predetermined electrical layout, with connectors attached. More complex circuit arrangements can also be made up using multiple layers of conductive ink patterns, with intervening dielectric layers as appropriate. Films thus prepared commonly are referred to as "static layers".
A simple or complex, multi-level static layer frequently is adhesively combined with a second film called a "graphic layer". The graphic layer includes the switch graphics printed on the top side of the film and a conductive circuit appropriately patterned, conductive printed on the back side. With this invention, the graphic layer can be direct printed or can be generated via transfers. The graphic layer and static layer are laminated with appropriate spacers, domes, tails, etc. to make up a complete, functioning flexible membrane circuit or switch.
The present invention is directed to improved components and techniques for the production of flexible membrane and similar circuits in a manner to greatly increase the variety and flexibility of circuit design and production. In particular, the present invention is directed to the provision of an electrically functional adhesive transfer product, in which circuit patterns for flexible membrane circuits are applied in one or more layers to a carrier sheet or film. The circuit elements are coated with a suitable pressure sensitive adhesive, which in turn is protected by a release paper. The circuit thus formed can later be applied to a desired substrate, by peeling away the protective release paper and by transferring and adhering the circuit to the substrate by the now exposed pressure sensitive adhesive. The carrier sheet, which can be peeled away after adhesive mounting of the circuit, advantageously is transparent, or at least translucent, so that the circuit pattern can be viewed through the carrier sheet to facilitate proper alignment with the substrate to which it is mounted.
A number of important advantages are derived from the invention. Among these, the electrically functional circuit can be transferred to a much wider variety of difficult substrates than is possible using conventional techniques. It is also possible to provide electrically functional adhesive transfer products that can be used to repair existing flexible membrane circuits. Additionally, it is contemplated that electrically functional adhesive transfer products may be made in a wide variety of "standard" components, such as connector pattern strips, dual in-line strips, insulator strips, elbows, doughnuts, terminal strips, etc. that can conveniently be used by circuit designers, to prepare and test prototype circuits, for example. This invention alternatively can be used to form electrically conductive adhesive transfers for such applications as electromagnetic interference (EMI), radio frequency interference (RFI) and electrostatic dissipating (ESD) in special shielding.
Other features and advantages of the invention will become apparent upon reference to the following detailed description of preferred embodiments and to the accompanying drawings.